LibJS/Bytecode: Reuse bytecode registers

This patch adds a register freelist to Bytecode::Generator and switches
all operands inside the generator to a new ScopedOperand type that is
ref-counted and automatically frees the register when nothing uses it.

This dramatically reduces the size of bytecode executable register
windows, which were often in the several thousands of registers for
large functions. Most functions now use less than 100 registers.

Userland/Libraries/LibJS/Bytecode/ScopedOperand.h

@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2024, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/RefCounted.h>
+#include <LibJS/Bytecode/Operand.h>
+
+namespace JS::Bytecode {
+
+class ScopedOperandImpl : public RefCounted<ScopedOperandImpl> {
+public:
+    ScopedOperandImpl(Generator& generator, Operand operand)
+        : m_generator(generator)
+        , m_operand(operand)
+    {
+    }
+
+    ~ScopedOperandImpl();
+
+    [[nodiscard]] Operand operand() const { return m_operand; }
+
+private:
+    Generator& m_generator;
+    Operand m_operand;
+};
+
+class ScopedOperand {
+public:
+    explicit ScopedOperand(Generator& generator, Operand operand)
+        : m_impl(adopt_ref(*new ScopedOperandImpl(generator, operand)))
+    {
+    }
+
+    [[nodiscard]] Operand operand() const { return m_impl->operand(); }
+    operator Operand() const { return operand(); }
+
+    [[nodiscard]] bool operator==(ScopedOperand const& other) const { return operand() == other.operand(); }
+
+private:
+    NonnullRefPtr<ScopedOperandImpl> m_impl;
+};
+
+}